Recently, there is an increasing demand for a lithium ion secondary battery, which is used in PDA, a portable electronic device, a home-use compact power storage facility, an auto-bicycle powered by a motor, an electric vehicle, a hybrid electric vehicle or the like.
In the above-mentioned lithium-ion batteries, an organic electrolyte is used as an electrolyte. Although an organic electrolyte has a high ionic conductivity, occurrence of leakage, ignition or the like to threaten the safety is concerned due to its nature of being liquid and flammable.
As a method for ensuring safety of a lithium ion secondary battery, an all-solid secondary battery in which an inorganic solid electrolyte is used instead of an organic electrolyte has been studied. However, in general, an inorganic solid electrolyte has a small ionic conductivity as compared with an organic electrolyte, and hence, practical application of an all-solid secondary battery was difficult.
As an inorganic solid electrolyte, a lithium ion-conductive ceramics based on Li3N has been reported. However, due to a low decomposition voltage, this ceramics could not be used in an all-solid secondary battery that is operated at a voltage of 3V or higher.
Non-Patent Document 1 discloses a solid electrolyte formed of a sulfide-based crystallized glass having a high lithium ion conductivity. However, the electrolyte disclosed in Non-Patent Document 1 is industrially disadvantageous since it requires a large amount of expensive germanium.
Patent Document 1 discloses that, in a glass sulfide-based solid electrolyte material that contains an ion conductor having an ortho composition and LiI and has a glass transition temperature, the ionic conductivity is increased to 1.0×10−3 S/cm.
In addition to those mentioned above, in the field of an all-solid battery, a sulfide-based solid electrolyte material has conventionally been used. For example, Patent Document 2 reports that glass ceramics electrolyte particles having a high ionic conductivity (˜2×10−3 S/cm) can be obtained by mixing Li2S and P2S5 at a specific molar ratio (68:32 to 73:27) and subjecting the mixture to mechanical milling, followed by a heat treatment. However, this material has a high reactivity and hence usage environment thereof is restricted.
Several methods have been proposed as a technology for suppressing this reactivity. The technology disclosed in Patent Document 3 has a problem that, while the reactivity is lowered, the ionic conductivity is significantly lowered.
Patent Document 1 discloses a solid electrolyte composed of sulfide-based crystallized glass having a high lithium ion conductivity. The electrolyte disclosed in Non-Patent Document 1 is industrially disadvantageous since a large amount of expensive lithium is required.
Further, Non-Patent Document 1 as mentioned above discloses that the ionic conductivity of the electrolyte is improved to 1.0×10−3 S/cm by addition of lithium iodide. However, further improvement in ionic conductivity has been required.